Golden Girl,
Jehovah's Witnesses believe that the "anointed class" of 144,000 persons (from Pentecost 44 CE to present day) was for all intents and purposes sealed as of the year 1935. In that year, the emphasis of the "harvest work" turned from the gathering of the heavenly elect to the ingathering of the "Jonadabs" (Great Crowd).
Traditionally, the teaching has been that some of the anointed (a "sizable" number) would STILL BE ALIVE at the onset of the Great Tribulation/Armageddon and actually pass through.
This belief is based on the scripture which says that "those days will be cut short for the sake of the Holy Ones."
Although I saw one comment in this thread that may suggest otherwise, I have not seen "officially" that the Society has revised or modified this view although it is only very rarely referenced in talks or publications.
Like the "Last Generation Doctrine" (discarded in 1995), this belief represents what I call "The Last Biological Clock Among Jehovah's Witnesses" and it presents a big problem to the Society as it clearly sets up the ticking clock for Armageddon to come before the approx. 8,000 anointed die from old age.
We can expect that something will have to give regarding this doctrine within the next 10 years or so.
The "Replacement theory" (which actually began as early as Russell's day and which he himself espoused) posits that some of the "bride" will cast away their "white robes" and fail to remain faithful. Since the 144,000 number is viewed by JWs as an actual number, there must be some replacements for these fallen ones.
Modernly, Jehovah's Witness use the above explanation to justify why some (often younger) persons sometimes suddenly claim to be anointed when they did not before.
-Eduardo